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<h1 class="title">creating torrents</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid&#64;rasterbar.com">arvid&#64;rasterbar.com</a></td></tr>
<tr><th class="docinfo-name">Version:</th>
<td>1.0.0</td></tr>
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<div class="contents topic" id="table-of-contents">
<p class="topic-title first">Table of contents</p>
<ul class="simple">
<li><a class="reference internal" href="#overview" id="id2">overview</a></li>
<li><a class="reference internal" href="#high-level-example" id="id3">high level example</a></li>
<li><a class="reference internal" href="#add-files" id="id4">add_files</a></li>
<li><a class="reference internal" href="#set-piece-hashes" id="id5">set_piece_hashes()</a></li>
<li><a class="reference internal" href="#file-storage" id="id6">file_storage</a><ul>
<li><a class="reference internal" href="#add-file" id="id7">add_file()</a></li>
<li><a class="reference internal" href="#hash-symlink-mtime-file-path-file-size-pad-file-at" id="id8">hash() symlink() mtime() file_path() file_size() pad_file_at()</a></li>
<li><a class="reference internal" href="#file-base-set-file-base" id="id9">file_base() set_file_base()</a></li>
</ul>
</li>
<li><a class="reference internal" href="#create-torrent" id="id10">create_torrent</a><ul>
<li><a class="reference internal" href="#id1" id="id11">create_torrent()</a></li>
<li><a class="reference internal" href="#generate" id="id12">generate()</a></li>
<li><a class="reference internal" href="#set-comment" id="id13">set_comment()</a></li>
<li><a class="reference internal" href="#set-creator" id="id14">set_creator()</a></li>
<li><a class="reference internal" href="#set-hash" id="id15">set_hash()</a></li>
<li><a class="reference internal" href="#set-file-hash" id="id16">set_file_hash()</a></li>
<li><a class="reference internal" href="#add-url-seed-add-http-seed" id="id17">add_url_seed() add_http_seed()</a></li>
<li><a class="reference internal" href="#add-node" id="id18">add_node()</a></li>
<li><a class="reference internal" href="#add-tracker" id="id19">add_tracker()</a></li>
<li><a class="reference internal" href="#set-root-cert" id="id20">set_root_cert()</a></li>
<li><a class="reference internal" href="#set-priv-priv" id="id21">set_priv() priv()</a></li>
<li><a class="reference internal" href="#merkle-tree" id="id22">merkle_tree()</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="overview">
<h1>overview</h1>
<p>This section describes the functions and classes that are used
to create torrent files. It is a layered API with low level classes
and higher level convenience functions. A torrent is created in 4
steps:</p>
<ol class="arabic simple">
<li>first the files that will be part of the torrent are determined.</li>
<li>the torrent properties are set, such as tracker url, web seeds,
DHT nodes etc.</li>
<li>Read through all the files in the torrent, SHA-1 all the data
and set the piece hashes.</li>
<li>The torrent is bencoded into a file or buffer.</li>
</ol>
<p>If there are a lot of files and or deep directoy hierarchies to
traverse, step one can be time consuming.</p>
<p>Typically step 3 is by far the most time consuming step, since it
requires to read all the bytes from all the files in the torrent.</p>
<p>All of these classes and functions are declared by including
<tt class="docutils literal">libtorrent/create_torrent.hpp</tt>.</p>
</div>
<div class="section" id="high-level-example">
<h1>high level example</h1>
<pre class="literal-block">
file_storage fs;

// recursively adds files in directories
add_files(fs, &quot;./my_torrent&quot;);

create_torrent t(fs);
t.add_tracker(&quot;http://my.tracker.com/announce&quot;);
t.set_creator(&quot;libtorrent example&quot;);

// reads the files and calculates the hashes
set_piece_hashes(t, &quot;.&quot;);

ofstream out(&quot;my_torrent.torrent&quot;, std::ios_base::binary);
bencode(std::ostream_iterator&lt;char&gt;(out), t.generate());
</pre>
</div>
<div class="section" id="add-files">
<h1>add_files</h1>
<blockquote>
<pre class="literal-block">
template &lt;class Pred&gt;
void add_files(file_storage&amp; fs, std::string const&amp; path, Pred p
        , boost::uint32_t flags = 0);
template &lt;class Pred&gt;
void add_files(file_storage&amp; fs, std::wstring const&amp; path, Pred p
        , boost::uint32_t flags = 0);

void add_files(file_storage&amp; fs, std::string const&amp; path
        , boost::uint32_t flags = 0);
void add_files(file_storage&amp; fs, std::wstring const&amp; path
        , boost::uint32_t flags = 0);
</pre>
</blockquote>
<p>Adds the file specified by <tt class="docutils literal">path</tt> to the <tt class="docutils literal">file_storage</tt> object. In case <tt class="docutils literal">path</tt>
refers to a diretory, files will be added recursively from the directory.</p>
<p>If specified, the predicate <tt class="docutils literal">p</tt> is called once for every file and directory that
is encountered. files for which <tt class="docutils literal">p</tt> returns true are added, and directories for
which <tt class="docutils literal">p</tt> returns true are traversed. <tt class="docutils literal">p</tt> must have the following signature:</p>
<pre class="literal-block">
bool Pred(std::string const&amp; p);
</pre>
<p>and for the wide string version:</p>
<pre class="literal-block">
bool Pred(std::wstring const&amp; p);
</pre>
<p>The path that is passed in to the predicate is the full path of the file or
directory. If no predicate is specified, all files are added, and all directories
are traveresed.</p>
<p>The &quot;..&quot; directory is never traversed.</p>
<p>The <tt class="docutils literal">flags</tt> argument should be the same as the flags passed to the <a class="reference internal" href="#create-torrent">create_torrent</a>
constructor.</p>
</div>
<div class="section" id="set-piece-hashes">
<h1>set_piece_hashes()</h1>
<blockquote>
<pre class="literal-block">
template &lt;class Fun&gt;
void set_piece_hashes(create_torrent&amp; t, std::string const&amp; p, Fun f);
template &lt;class Fun&gt;
void set_piece_hashes(create_torrent&amp; t, std::wstring const&amp; p, Fun f);
template &lt;class Fun&gt;
void set_piece_hashes(create_torrent&amp; t, std::string const&amp; p, Fun f
        , error_code&amp; ec);
template &lt;class Fun&gt;
void set_piece_hashes(create_torrent&amp; t, std::wstring const&amp; p, Fun f
        , error_code&amp; ec);

void set_piece_hashes(create_torrent&amp; t, std::string const&amp; p);
void set_piece_hashes(create_torrent&amp; t, std::wstring const&amp; p);
void set_piece_hashes(create_torrent&amp; t, std::string const&amp; p
        , error_code&amp; ec);
void set_piece_hashes(create_torrent&amp; t, std::wstring const&amp; p
        , error_code&amp; ec);
</pre>
</blockquote>
<p>This function will assume that the files added to the torrent file exists at path
<tt class="docutils literal">p</tt>, read those files and hash the content and set the hashes in the <tt class="docutils literal">create_torrent</tt>
object. The optional function <tt class="docutils literal">f</tt> is called in between every hash that is set. <tt class="docutils literal">f</tt>
must have the following signature:</p>
<pre class="literal-block">
void Fun(int);
</pre>
<p>The overloads that don't take an <tt class="docutils literal">error_code&amp;</tt> may throw an exception in case of a
file error, the other overloads sets the error code to reflect the error, if any.</p>
</div>
<div class="section" id="file-storage">
<h1>file_storage</h1>
<p>The <tt class="docutils literal">file_storage</tt> class represents a file list and the piece
size. Everything necessary to interpret a regular bittorrent storage
file structure. Its synopsis:</p>
<pre class="literal-block">
class file_storage
{
public:

        bool is_valid() const;

        enum flags_t
        {
                pad_file = 1,
                attribute_hidden = 2,
                attribute_executable = 4
        };

        void add_file(file_entry const&amp; e);
        void add_file(std::string const&amp; p, size_type size, int flags = 0);
        void add_file(std::wstring const&amp; p, size_type size, int flags = 0);
        void rename_file(int index, std::string const&amp; new_filename);
        void rename_file(int index, std::wstring const&amp; new_filename);

        std::vector&lt;file_slice&gt; map_block(int piece, size_type offset
                , int size) const;
        peer_request map_file(int file, size_type offset, int size) const;

        typedef std::vector&lt;internal_file_entry&gt;::const_iterator iterator;
        typedef std::vector&lt;internal_file_entry&gt;::const_reverse_iterator reverse_iterator;

        iterator begin() const;
        iterator end() const;
        reverse_iterator rbegin();
        reverse_iterator rend() const;
        int num_files() const;

        file_entry at(int index) const;

        size_type total_size() const;
        void set_num_pieces(int n);
        int num_pieces() const;
        void set_piece_length(int l);
        int piece_length() const;
        int piece_size(int index) const;

        // index accessors
        sha1_hash const&amp; hash(int index) const;
        std::string const&amp; symlink(int index) const;
        time_t mtime(int index) const;
        size_type file_base(int index) const;
        void set_file_base(int index, size_type off);
        std::string file_path(int index) const;
        std::string file_name(int index) const;
        size_type file_size(int index) const;
        bool pad_file_at(int index) const;
        size_type file_offset(int index) const;

        // iterator accessors
        sha1_hash hash(internal_file_entry const&amp; fe) const;
        std::string const&amp; symlink(internal_file_entry const&amp; fe) const;
        time_t mtime(internal_file_entry const&amp; fe) const;
        int file_index(internal_file_entry const&amp; fe) const;
        size_type file_base(internal_file_entry const&amp; fe) const;
        void set_file_base(internal_file_entry const&amp; fe, size_type off);
        std::string file_path(internal_file_entry const&amp; fe) const;
        std::string file_name(internal_file_entry const&amp; fe) const;
        size_type file_size(internal_file_entry const&amp; fe) const;
        bool pad_file_at(internal_file_entry const&amp; fe) const;
        size_type file_offset(internal_file_entry const&amp; fe) const;

        void set_name(std::string const&amp; n);
        void set_name(std::wstring const&amp; n);
        const std::string&amp; name() const;

        void swap(file_storage&amp; ti);
}
</pre>
<div class="section" id="add-file">
<h2>add_file()</h2>
<blockquote>
<pre class="literal-block">
void add_file(file_entry const&amp; e);
void add_file(std::string const&amp; p, size_type size, int flags = 0);
void add_file(std::wstring const&amp; p, size_type size, int flags = 0);
</pre>
</blockquote>
<p>Adds a file to the file storage. The <tt class="docutils literal">flags</tt> argument sets attributes on the file.
The file attributes is an extension and may not work in all bittorrent clients.
The possible arreibutes are:</p>
<pre class="literal-block">
pad_file
attribute_hidden
attribute_executable
</pre>
<p>If more files than one are added, certain restrictions to their paths apply.
In a multi-file file storage (torrent), all files must share the same root directory.</p>
<p>That is, the first path element of all files must be the same.
This shared path element is also set to the name of the torrent. It
can be changed by calling <tt class="docutils literal">set_name</tt>.</p>
<p>The built in functions to traverse a directory to add files will
make sure this requirement is fulfilled.</p>
</div>
<div class="section" id="hash-symlink-mtime-file-path-file-size-pad-file-at">
<h2>hash() symlink() mtime() file_path() file_size() pad_file_at()</h2>
<blockquote>
<pre class="literal-block">
sha1_hash hash(int index) const;
std::string const&amp; symlink(int index) const;
time_t mtime(int index) const;
std::string file_path(int index) const;
size_type file_size(int index) const;
bool pad_file_at(int index) const;
</pre>
</blockquote>
<p>These functions are used to query attributes of files at
a given index.</p>
<p>The <tt class="docutils literal">file_hash()</tt> is a sha-1 hash of the file, or 0 if none was
provided in the torrent file. This can potentially be used to
join a bittorrent network with other file sharing networks.</p>
<p>The <tt class="docutils literal">mtime()</tt> is the modification time is the posix
time when a file was last modified when the torrent
was created, or 0 if it was not included in the torrent file.</p>
<p><tt class="docutils literal">file_path()</tt> returns the full path to a file.</p>
<p><tt class="docutils literal">file_size()</tt> returns the size of a file.</p>
<p><tt class="docutils literal">pad_file_at()</tt> returns true if the file at the given
index is a pad-file.</p>
</div>
<div class="section" id="file-base-set-file-base">
<h2>file_base() set_file_base()</h2>
<blockquote>
<pre class="literal-block">
size_type file_base(int index) const;
void set_file_base(int index, size_type off);
</pre>
</blockquote>
<p>The file base of a file is the offset within the file on the filsystem
where it starts to write. For the most part, this is always 0. It's
possible to map several files (in the torrent) into a single file on
the filesystem by making them all point to the same filename, but with
different file bases, so that they don't overlap.
<tt class="docutils literal"><span class="pre">torrent_info::remap_files</span></tt> can be used to use a new file layout.</p>
</div>
</div>
<div class="section" id="create-torrent">
<h1>create_torrent</h1>
<p>The <tt class="docutils literal">create_torrent</tt> class has the following synopsis:</p>
<pre class="literal-block">
struct create_torrent
{
        enum {
                optimize = 1
                , merkle = 2
                , modification_time = 4
                , symlinks = 8
                , calculate_file_hashes = 16
        };
        create_torrent(file_storage&amp; fs, int piece_size = 0, int pad_size_limit = -1
                , int flags = optimize, int alignment = 0x4000);
        create_torrent(torrent_info const&amp; ti);

        entry generate() const;

        file_storage const&amp; files() const;

        void set_comment(char const* str);
        void set_creator(char const* str);
        void set_hash(int index, sha1_hash const&amp; h);
        void set_file_hash(int index, sha1_hash const&amp; h);
        void add_url_seed(std::string const&amp; url);
        void add_http_seed(std::string const&amp; url);
        void add_node(std::pair&lt;std::string, int&gt; const&amp; node);
        void add_tracker(std::string const&amp; url, int tier = 0);
        void set_root_cert(std::string const&amp; pem);
        void set_priv(bool p);

        int num_pieces() const;
        int piece_length() const;
        int piece_size(int i) const;
        bool priv() const;
};
</pre>
<div class="section" id="id1">
<h2>create_torrent()</h2>
<blockquote>
<pre class="literal-block">
enum {
        optimize = 1
        , merkle = 2
        , modification_time = 4
        , symlinks = 8
        , calculate_file_hashes = 16
};
create_torrent(file_storage&amp; fs, int piece_size = 0, int pad_size_limit = -1
        , int flags = optimize, int alignment = 0x4000);
create_torrent(torrent_info const&amp; ti);
</pre>
</blockquote>
<p>The <tt class="docutils literal">piece_size</tt> is the size of each piece in bytes. It must
be a multiple of 16 kiB. If a piece size of 0 is specified, a
piece_size will be calculated such that the torrent file is roughly 40 kB.</p>
<p>If a <tt class="docutils literal">pad_size_limit</tt> is specified (other than -1), any file larger than
the specified number of bytes will be preceeded by a pad file to align it
with the start of a piece. The pad_file_limit is ignored unless the
<tt class="docutils literal">optimize</tt> flag is passed. Typically it doesn't make sense to set this
any lower than 4kiB.</p>
<p>The overload that takes a <tt class="docutils literal">torrent_info</tt> object will make a verbatim
copy of its info dictionary (to preserve the info-hash). The copy of
the info dictionary will be used by <tt class="docutils literal">generate()</tt>. This means
that none of the member functions of create_torrent that affects
the content of the info dictionary (such as <tt class="docutils literal">set_hash()</tt>), will
have any affect.</p>
<p>The <tt class="docutils literal">flags</tt> arguments specifies options for the torrent creation. It can
be any combination of the following flags:</p>
<dl class="docutils">
<dt>optimize</dt>
<dd>This will insert pad files to align the files to piece boundaries, for
optimized disk-I/O.</dd>
<dt>merkle</dt>
<dd>This will create a merkle hash tree torrent. A merkle torrent cannot
be opened in clients that don't specifically support merkle torrents.
The benefit is that the resulting torrent file will be much smaller and
not grow with more pieces. When this option is specified, it is
recommended to have a fairly small piece size, say 64 kiB.
When creating merkle torrents, the full hash tree is also generated
and should be saved off separately. It is accessed through  the
<tt class="docutils literal">merkle_tree()</tt> function.</dd>
<dt>modification_time</dt>
<dd>This will include the file modification time as part of the torrent.
This is not enabled by default, as it might cause problems when you
create a torrent from separate files with the same content, hoping to
yield the same info-hash. If the files have different modification times,
with this option enabled, you would get different info-hashes for the
files.</dd>
<dt>symlink</dt>
<dd>If this flag is set, files that are symlinks get a symlink attribute
set on them and their data will not be included in the torrent. This
is useful if you need to reconstruct a file hierarchy which contains
symlinks.</dd>
<dt>calculate_file_hashes</dt>
<dd>If this is set, the <a class="reference internal" href="#set-piece-hashes">set_piece_hashes()</a> function will, as it calculates
the piece hashes, also calculate the file hashes and add those associated
with each file. Note that unless you use the <a class="reference internal" href="#set-piece-hashes">set_piece_hashes()</a> function,
this flag will have no effect.</dd>
</dl>
<p><tt class="docutils literal">alignment</tt> is used when pad files are enabled. This is the size eligible
files are aligned to. The default is the default bittorrent block size of
16 kiB. It is common to align to the piece size of the torrent.</p>
</div>
<div class="section" id="generate">
<h2>generate()</h2>
<blockquote>
<pre class="literal-block">
entry generate() const;
</pre>
</blockquote>
<p>This function will generate the .torrent file as a bencode tree. In order to
generate the flat file, use the bencode() function.</p>
<p>It may be useful to add custom entries to the torrent file before bencoding it
and saving it to disk.</p>
<p>If anything goes wrong during torrent generation, this function will return
an empty <tt class="docutils literal">entry</tt> structure. You can test for this condition by querying the
type of the entry:</p>
<pre class="literal-block">
file_storage fs;
// add file ...
create_torrent t(fs);
// add trackers and piece hashes ...
e = t.generate();

if (e.type() == entry::undefined_t)
{
        // something went wrong
}
</pre>
<p>For instance, you cannot generate a torrent with 0 files in it. If you don't add
any files to the <tt class="docutils literal">file_storage</tt>, torrent generation will fail.</p>
</div>
<div class="section" id="set-comment">
<h2>set_comment()</h2>
<blockquote>
<pre class="literal-block">
void set_comment(char const* str);
</pre>
</blockquote>
<p>Sets the comment for the torrent. The string <tt class="docutils literal">str</tt> should be utf-8 encoded.
The comment in a torrent file is optional.</p>
</div>
<div class="section" id="set-creator">
<h2>set_creator()</h2>
<blockquote>
<pre class="literal-block">
void set_creator(char const* str);
</pre>
</blockquote>
<p>Sets the creator of the torrent. The string <tt class="docutils literal">str</tt> should be utf-8 encoded.
This is optional.</p>
</div>
<div class="section" id="set-hash">
<h2>set_hash()</h2>
<blockquote>
<pre class="literal-block">
void set_hash(int index, sha1_hash const&amp; h);
</pre>
</blockquote>
<p>This sets the SHA-1 hash for the specified piece (<tt class="docutils literal">index</tt>). You are required
to set the hash for every piece in the torrent before generating it. If you have
the files on disk, you can use the high level convenience function to do this.
See <a class="reference internal" href="#set-piece-hashes">set_piece_hashes()</a>.</p>
</div>
<div class="section" id="set-file-hash">
<h2>set_file_hash()</h2>
<blockquote>
<pre class="literal-block">
void set_file_hash(int index, sha1_hash const&amp; h);
</pre>
</blockquote>
<p>This sets the sha1 hash for this file. This hash will end up under the key <tt class="docutils literal">sha1</tt>
associated with this file (for multi-file torrents) or in the root info dictionary
for single-file torrents.</p>
</div>
<div class="section" id="add-url-seed-add-http-seed">
<h2>add_url_seed() add_http_seed()</h2>
<blockquote>
<pre class="literal-block">
void add_url_seed(std::string const&amp; url);
void add_http_seed(std::string const&amp; url);
</pre>
</blockquote>
<p>This adds a url seed to the torrent. You can have any number of url seeds. For a
single file torrent, this should be an HTTP url, pointing to a file with identical
content as the file of the torrent. For a multi-file torrent, it should point to
a directory containing a directory with the same name as this torrent, and all the
files of the torrent in it.</p>
<p>The second function, <tt class="docutils literal">add_http_seed()</tt> adds an HTTP seed instead.</p>
</div>
<div class="section" id="add-node">
<h2>add_node()</h2>
<blockquote>
<pre class="literal-block">
void add_node(std::pair&lt;std::string, int&gt; const&amp; node);
</pre>
</blockquote>
<p>This adds a DHT node to the torrent. This especially useful if you're creating a
tracker less torrent. It can be used by clients to bootstrap their DHT node from.
The node is a hostname and a port number where there is a DHT node running.
You can have any number of DHT nodes in a torrent.</p>
</div>
<div class="section" id="add-tracker">
<h2>add_tracker()</h2>
<blockquote>
<pre class="literal-block">
void add_tracker(std::string const&amp; url, int tier = 0);
</pre>
</blockquote>
<p>Adds a tracker to the torrent. This is not strictly required, but most torrents
use a tracker as their main source of peers. The url should be an <a class="reference external" href="http://">http://</a> or udp://
url to a machine running a bittorrent tracker that accepts announces for this torrent's
info-hash. The tier is the fallback priority of the tracker. All trackers with tier 0 are
tried first (in any order). If all fail, trackers with tier 1 are tried. If all of those
fail, trackers with tier 2 are tried, and so on.</p>
</div>
<div class="section" id="set-root-cert">
<h2>set_root_cert()</h2>
<blockquote>
<pre class="literal-block">
void set_root_cert(std::string const&amp; pem);
</pre>
</blockquote>
<p>This function sets an X.509 certificate in PEM format to the torrent. This makes the
torrent an <em>SSL torrent</em>. An SSL torrent requires that each peer has a valid certificate
signed by this root certificate. For SSL torrents, all peers are connecting over SSL
connections. For more information on SSL torrents, see the <a class="reference external" href="manual.html#ssl-torrents">manual</a>.</p>
<p>The string is not the path to the cert, it's the actual content of the certificate,
loaded into a std::string.</p>
</div>
<div class="section" id="set-priv-priv">
<h2>set_priv() priv()</h2>
<blockquote>
<pre class="literal-block">
void set_priv(bool p);
bool priv() const;
</pre>
</blockquote>
<p>Sets and queries the private flag of the torrent.</p>
</div>
<div class="section" id="merkle-tree">
<h2>merkle_tree()</h2>
<blockquote>
<pre class="literal-block">
std::vector&lt;sha1_hash&gt; const&amp; merkle_tree() const;
</pre>
</blockquote>
<p>This function returns the merkle hash tree, if the torrent was created as a merkle
torrent. The tree is created by <tt class="docutils literal">generate()</tt> and won't be valid until that function
has been called. When creating a merkle tree torrent, the actual tree itself has to
be saved off separately and fed into libtorrent the first time you start seeding it,
through the <tt class="docutils literal"><span class="pre">torrent_info::set_merkle_tree()</span></tt> function. From that point onwards, the
tree will be saved in the resume data.</p>
</div>
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